1. Field of the Invention
The present invention relates to a pattern forming method and, more particularly, to a method of forming a resist pattern applied to micropatterning in fabricating electronic components such as large-scale semiconductor integrated circuits (LSIs).
2. Description of the Related Art
In the fabrication of various electronic components including semiconductor integrated circuits, micropatterning techniques using photolithography are adopted. In practice, such techniques are carried out by the following process. First, a photoresist film is formed on a semiconductor substrate by means of, e.g., a spin coating method. After the resist film is patternexposed, development and rinsing are performed to form a desired resist pattern. Subsequently, this resist pattern is used as an anti-etching mask to etch the substrate, thereby forming fine lines or opening portions. Especially in recent years, to increase the degree of integration along with an increase in packaging density of electronic components, it is required that very fine resist patterns on a submicron order be formed in the above process.
As an exposure apparatus for forming the resist pattern, a reduction-projecting mask aligner of a stepand-repeat type, generally called a stepper, is used. Examples of a radiation source for use in this stepper are g-line (wavelength 436 nm), h-line (wavelength 405 nm), and i-line (wavelength 365 nm) of mercury lamps, and excimer lasers, such as XeF (wavelength 351 nm), XeCl (wavelength 308 nm), KrF (wavelength 248 nm), KrCl (wavelength 222 nm), ArF (wavelength 193 nm), and F.sub.2 (wavelength 157 nm). To form fine patterns, the wavelength of a radiation source used is preferably as short as possible. In particular, deep Uv of an excimer laser or the like is preferable. It is also possible to form finer patterns by using electron beam or X-ray with a shorter wavelength.
Since, however, a conventional resist has a large absorption for deep UV, such radiation cannot satisfactorily reach a position deep in a resist film. That is, a chemical change occurring upon exposure to deep UV cannot progress sufficiently in a region deep from the surface of a resist film. Consequently, the sectional shape of a resist pattern formed becomes a triangle, for a positive resist, or an inverted triangle, for a negative resist. This significantly degrades the function of the resist pattern as an anti-etching mask.
To solve this problem, the use of a resist called a chemical amplification-type resist has been proposed. The chemical amplification-type resist is, for example, a photosensitive composition containing a compound which generates an acid when exposed to light, i.e., a photo-acid generator, and a compound, the hydrophobic group of which is decomposed by the generated acid, and which therefore changes into a hydrophilic compound. Practical examples are a positive resist containing a polymer, in which a hydroxyl group of poly(p-hydroxystyrene) is blocked by butoxycarbonyl group, and an onium salt as the photo-acid generator, disclosed in H. Ito, C. G. Wilson, and J. M. J. Frechet, U.S. Pat. No. 4,491,628 (1985); and a positive resist containing a m-cresol novolak resin, nathphalene-2-carboxylic acid-tert-butyl ester, and, as the photo-acid generator, triphenylsulfonium salt, described in M. J. O'Brien, J. V. Crivello, SPIE Vol. 920, Advances in Resist Technology and Processing, p. 42, (1988). In addition, a positive resist containing 2,2-bis(4-tert-butoxycarbonyloxyphenyl)propane or polyphthalaldehyde, and an onium salt as the photo-acid generator, is described in H. Ito, SPIE Vol. 920, Advances in Resist Technology and Processing, p. 33, (1988).
In these chemical amplification-type resists, the acid generated by the photo acid-generator functions as a catalyst. Hence, only a small amount of the acid can efficiently induce a chemical change inside the resist. As a result, when the resist film is exposed to a radiation, the reaction can sufficiently proceed even in a portion inside the film which the radiation is difficult to reach compared to the film surface. This makes it possible to form a resist pattern with steep side surfaces in line portions upon development.
The chemical amplification-type resist described above, however, has a high sensitivity and is therefore readily influenced by oxygen, moisture, and other minor contaminations in a process atmosphere. As an example, S. A. MacDonald et al., SPIE Vol. 1,466, Advances in Resist Technology and Processing, p. 2, (1991) reports that a trace amount of dimethylaniline contained in an atmosphere deactivates an acid generated in the vicinity of the surface of a chemical amplification-type resist film when exposed to light to produce a surface inhibition layer, i.e., a layer with a significantly low dissolving rate into a developing solution on the surface of the resist film, and this surface inhibition layer remains like eaves on the surface of a resist pattern after exposure and development. The surface inhibition layer decreases the resolution of the resist, and the eaves produced on the resist pattern by that layer adversely affect the etching accuracy in a region of a semiconductor substrate.
various methods have been proposed in order to solve the above problems.
For example, a method of improving the photo-acid generator contained in the chemical amplification-type resist to increase the concentration of an acid to be generated has been attempted. In this method, however, if the generated acid is a strong acid, it is difficult to control the acid. If the generated acid is a weak acid, the strength of the acid is insufficient although its concentration is high. In either case, the result is a reduction in resolution of a resultant pattern. There is another attempt to add a basic compound, as an additive, to the resist, thereby relatively relaxing the influence of basic substances in an atmosphere on the resist film, or to add an acidic compound to neutralize the basic substances from an atmosphere. However, the effect of either method is still unsatisfactory.
In addition, a method of forming a protective film on a film of the chemical amplification-type resist to shield the film surface from an atmosphere has been proposed. For instance, Published Unexamined Japanese Patent Application No. 63-287950 describes a method of stacking a neutral polymer, such as polyvinyl alcohol or polyvinyl pyrrolidone, as a protective film on a chemical amplification-type printing plate to prolong the storage life of the printing plate. However, although polyvinyl alcohol or polyvinyl pyrrolidone as the material of the protective film has a barrier effect against oxygen or moisture, it cannot always effectively prevent the formation of the surface inhibition layer described above.